A forklift is provided with a travel device and a working machine. The travel device is driven by an engine. The working machine has the engine as a drive source and operates when a pressure oil discharged from a working machine hydraulic pump is supplied to a working machine hydraulic actuator via a control valve.
Here, the control valve comprises a lift control valve, a tilt control valve, and an attachment control valve. The working machine hydraulic actuator comprises a lift cylinder, a tilt cylinder, and an attachment cylinder.
The forklift is provided with working machine operating means for operating the working machine according to the operation. The working machine operating means is operated in the direction of raising the lift so that the lift cylinder of the working machine operates in the direction of raising the lift, and the working machine operating means is operated in the tilt operation direction so that the working machine operates in the tilt direction.
The forklift is provided with an engine control means. The engine control means controls the engine so as to have an engine rotation speed according to the accelerator pedal operation with the high idle rotation speed as an upper limit value.
A maximum supply flow rate supplied from the working machine hydraulic pump to the working machine hydraulic actuator is set to be a flow rate required to operate the lift cylinder at the maximum speed in the direction of raising the lift.
On the other hand, in the case where the tilt cylinder only is independently operated in the tilt direction, it is not necessary to discharge the maximum supply flow rate from the working machine hydraulic pump, and it is sufficient to have about 50% of the maximum supply capacity of the working machine hydraulic pump. That is, the tilt cylinder can be operated at the maximum speed in the tilt direction by about a half of the flow rate that is discharged when the lift cylinder is operated at the maximum speed in the direction of raising the lift.
Here, for the working machine hydraulic pump, there is used a fixed displacement type hydraulic pump or a variable displacement type hydraulic pump.
When the fixed displacement type hydraulic pump is used as the working machine hydraulic pump, the flow rate to supply to the working machine hydraulic actuator is determined according to an opening area of the control valve. Thus, the opening area of the tilt control valve is set to be smaller than the opening area of the lift control valve to supply a necessary flow rate to the tilt cylinder.
Meanwhile, when the variable displacement type hydraulic pump is used as the working machine hydraulic pump, it is general to control a capacity of the working machine hydraulic pump such that a pressure difference before and after the control valve becomes a constant value (constant differential pressure control). By this constant differential pressure control, the operating oil having a flow rate corresponding to the opening area of the control valve is supplied to the working machine hydraulic actuator without depending on the load of the working machine. Therefore, in the case where the tilt cylinder is operated in the tilt direction, constant forward-and-backward control is performed to decrease the capacity of the working machine hydraulic pump, and the tilt cylinder is supplied with a necessary flow rate.
As a patent reference related to the present invention, there is the following Patent Reference 1.
The following Patent Reference 1 discloses an invention that aims to limit the maximum speed at the time when an inching pedal is depressed and, when the inching pedal of a forklift having its engine as a drive source is depressed, limits the maximum rotation speed of the engine to a rotation speed lower than an allowable maximum rotation speed. And, Patent Reference 1 also discloses an invention that aims to prevent the forklift from causing collapse of cargo while driving and, when the lift cylinder pressure is under no load, limits the engine maximum rotation speed of the forklift having the engine as a drive source to a rotation speed lower than the allowable maximum rotation speed.    Patent Reference 1: Japanese Patent Application Laid-Open No. 2010-71095